Rapid and efficient adsorption of p-nitrophenol over biomass-derived vertically aligned graphene nanosheets fabricated by hydrothermal/molten salt-assisted pyrolysis method

Abstract

Vertical graphene (VG) is an emerging three-dimensional graphene with unique structure and properties, which is expected to possess outstanding adsorption performance. Nevertheless, the utilization of VG as an adsorbent has not been investigated, and its conventional preparation methods are complex and expensive. Herein, we utilize a facile hydrothermal/ molten K2CO3-assisted pyrolysis method for preparing VG from cost-effective and renewable fir bark. The optimized VG-1.0 achieved a high adsorption capacity (556 mg/g) for p-nitrophenol (PNP) within 10 min, far superior to commercial activated carbon (360 mg/g) and previously reported graphene-based materials. Its excellent performance can be ascribed to high specific surface area (1423 m2/g), open & vertical channels, rich oxygen-containing groups and high graphitization degree, providing sufficient adsorption sites and unhindered mass transfer pathways. VG-1.0 was easily regenerated by rinsing with NaOH solution and showed good reusability, maintaining > 80 % of its original adsorption capacity after 5 cycles of reuse. The adsorption isotherm and kinetic data of PNP over VG were fitted well with Freundlich/Temkin model and pseudo-second-order kinetic model, respectively. Various characterizations and density functional theory (DFT) simulations have confirmed that the adsorption of PNP on VG is a spontaneous and endothermic process, involving pore filling and surface adsorption such as hydrogen bonding and π-π interactions.